When ‘Oumuamua was first detected on October 19th, 2017, astronomers were understandably confused about the nature of this strange object. Initially thought to be an interstellar comet, it was then designated as an interstellar asteroid. But when it picked up velocity as it departed our Solar System (a very comet-like thing to do), scientists could only scratch their heads and wonder.
After much consideration, Shmuel Bialy and Professor Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA) proposed that ‘Oumuamua could in fact be an artificial object (possibly an alien probe). In a more recent study, Amir Siraj and Prof. Loeb identified another (and much smaller) potential interstellar object, which they claim could be regularly colliding with Earth.
The study, “Discovery of a Meteor of Interstellar Origin“, recently appeared online and was submitted for publication in The Astrophysical Journal Letters. In it, Siraj and Loeb expand upon previous research they conducted which indicated that there is an abundance of interstellar objects in the Solar System that could be researched.
However, for the sake of this study, Siraj and Loeb chose to focus on meter-sized interstellar objects that made their way into our Solar System over time. Many of these could have found there way into Earth’s atmosphere as meteorites, presenting humanity with the opportunity to study objects that come from extrasolar systems. As Prof. Loeb shared with Universe Today via email:
“This is a new way to learn about interstellar objects. The traditional search method uses the Sun as a lamppost and searches for objects based on their reflected sunlight. This is how`Oumuamua was detected by Pan STARRS, which is effective for objects larger than 100 meters in size. One expects many more objects of smaller size, some of which will hit the Earth.”
To determine how often meter-sized objects enter our Solar System and/or collide with Earth, Siraj and Loeb analyzed data from the Center for Near Earth Objects (CNEOS), which is tasked with monitoring the orbits of asteroids and comets to determine if they will ever impact Earth. Specifically, they were looking for particularly bright and explosive events (bolides) from the past three decades.
These events have become the focus of considerable attention ever since the Chelyabinsk meteor exploded in the skies over a small Russian town in 2013. And with the recent meteor that exploded above the Bering Sea in December of 2018 – which was observed by the NASA Terra satellite – Prof. Loeb was inspired to examine the CNEOS catalog to determine how common these types of bolide events are.
“About two weeks ago I had a radio interview in which I was asked about a meteor that was seen above the Bering Sea in December 2018,” said Loeb. “In preparation for this interview I read the literature on meteors and found the catalog of all meteors over the past three decades. I then asked an undergraduate student working with me, Amir Siraj, to integrate the orbits of the fastest meteors back in time taking account of the gravity of the Earth, the Sun and all other planets in the Solar System, using the three components of velocity, position and time of impact [for] the meteors.”
After looking into three decades of meteorites, they discovered one bolide event which could very-well have been the result of an interstellar meteor entering Earth’s atmosphere. This meteor was spotted just north of Manus Island, off the coast of Papua New Guinea, on January 8th, 2014, and measured an estimated 1 meter (3.28 ft) in diameter, with a mass of 500 kg (1100 lbs).
Based on the object’s size, motion, and velocity – 60 km/s (37 mi/s) relative to Earth’s motion – they determined that the meteor is likely to have been interstellar in nature. Based on its likely origin, this discovery could have profound implications concerning the study of how life originated here on Earth. As Loeb explained:
“Such a high ejection speed can only be produced in the innermost cores of planetary system (interior to the orbit of the Earth around a star like the Sun, but in the habitable zone of dwarf stars – hence allowing such objects to carry life from their parent planets).
Aside from constraining this meteor’s origin, Siraj and Leob also calculated just how often such objects would impact Earth (once per decade) and how often they would need to be ejected from their respective systems in order for some to make it to other stars. While the numbers were rather (ahem!) astronomical, they found that the necessary mass of ejected meter-sized objects was the same as ejected ‘Oumuamua-sized objects (100 m; 328 feet).
“Altogether, each star needs to eject about 10^{22} objects of 1 meter size to account for the population of this meteor,” said Loeb. “This is roughly the total number of stars in the observable volume of the Universe… Each star needs to eject about an Earth mass of rocks with this mass, which is challenging because this is the total mass in planetesimals inferred in the appropriate inner region of the early Solar System.”
Beyond the implications this study could have for the spread of life throughout the cosmos (aka. panspermia) and the abundance of interstellar objects in our Solar System (and others), this study presents a new detection method from which it will be possible to infer the composition of interstellar objects. The way to do this, said Loeb, is to conduct spectral analyses of the gases they leave after they burn up in our atmosphere:
“In the future, astronomers can establish an alert system that triggers spectroscopic observations by the nearest telescope for meteors of a possible interstellar origin. We already have alert systems for gravitational wave sources, gamma-ray bursts, or fast radio bursts.”
This echoes suggestions made by Dr. Zdenek Sekanina of the NASA Jet Propulsion Laboratory, who recently conducted a study that claimed that ‘Oumuamua could be the remains of an interstellar comet that broke up as it approached out Sun. As Sekanina argued, examining the spectra of the dust left behind after the comet exploded would reveal things about the system in which the comet originally formed.
While this alert system would admittedly only detect a small percentage of interstellar meteors entering our atmosphere, the scientific payoff of studying them would be immeasurable. At the very least, we will be able to learn things about distant star systems without having to actually send missions there. At most, there’s the remote possibility that one or more of these meteors could be space junk from another civilization.
Imagine what we could learn if that were the case!
Further Reading: arXiv